Field of Invention
The present invention belongs to a technical field of optical detection, and particularly relates to a method and a device for reducing the extrinsic dark count of a superconducting nanowire single photon detector.
Description of Related Arts
A superconducting nanowire single photon detector (SNSPD) is an important photodetector, which may achieve single photon detection from a visible wavelength band to an infrared wavelength band. The SNSPD mainly adopts a low temperature superconducting ultrathin film material, e.g., NbN, Nb, TaN, NbTiN, WSi and the like, with a typical thickness of 5-10 nm; the device usually adopts a zigzag nanowire with a width of about 100 nm. An existing superconducting nanowire single photon detector structure is shown in FIG. 1, comprising substrates 20-40 having an upper surface and a lower surface with an anti-reflection layer respectively, an optical cavity structure 50, a superconducting nanowire 60 and a reflector 70, etc.
When in use, the SNSPD is placed in a low temperature environment (<4 k), and the device is in the superconductive state, further, the device is applied with a certain bias current Ib, which is slightly less than the critical current Ic of the device. Once single photons incident on the nanowire of the device, cooper-pairs are broken for generating a large amount of thermal electrons to form a local hotspot, which is further diffused due to Joule heat under the action of the bias current Ib, with the result that a part of the nanowire is quenched to form a resistance area. After that, the thermal electrons are transferred and relaxed by electron-phonon interactions, and are further re-paired as cooper-pairs of superconducting state. Since the superconducting material has a short thermal relaxation time, after the SNSPD receives a single photon, it may generate fast electrical pulse signals at two ends of the device, so as to achieve the detection function of the single photons.
Dark count is one of the main parameters of the single photon detector. It refers to the error count that is irrelevant to signale photons. The dark count of the SNSPD is derived from two aspects: one is the dark count caused by a magnetic vortex motion of the SNSPD nanowire, which is called as an intrinsic dark count; the intrinsic dark count is relevant to the working current of the device and is generated only if the working current is very close to the critical current, with a counting rate thereof being exponentially dependent on the bias current. The count of the SNSPD triggered by other non-signal photons are commonly called as the extrinsic dark count, which may come from probabilities as below: (1) the dark count caused by thermal radiation of the optical fiber material per se; (2) when the SNSPD is in use, a small amount of various optical (heat) radiations in the working environment may enter the optical fiber through a coating layer thereof as a SNSPD count triggered by stray light. The extrinsic dark count may equivalent to a certain amount of photon radiation, with an introduced dark count being proportional to detection efficiency of the detector. The dark count is crucial to many applications of single photon detections, especially, the level of the dark count is a key parameter determining signal-to-noise ratio of coding and communication distance as for a long-distance fiber quantum communication. Currently, there is no efficient method for thoroughly solving the intrinsic dark count, and it usually adopts an approach reducing the bias current of the SNSPD in the prior art. Under such conditions, the extrinsic dark count plays a decisive role. Shibata et. al in Japan have proposed a method that introduces an optical fiber filter in an optical fiber at low temperature, which may efficiently reduce the extrinsic dark count, but still, significantly attenuate signal light (for about 3 dB), which directly affects the detection efficiency of the device.